Method for producing magnesium and chlorine and electrolytic cell for implementing same

ABSTRACT

The invention relates to producing magnesium and chlorine from a solution of magnesium chloride-containing salts, using an electrolytic cell. A diaphragmless electrolytic cell includes: an electrolysis chamber with alternating anodes and cathodes; and a magnesium separation cell separated from the electrolysis chamber by a partition having upper V-shaped circulation channels and lower circulation channels. Electrolysis is carried out at 6-25 gas saturation of the electrolyte with chlorine bubbles in an interelectrode gap. The flow rate of the electrolyte in the upper circulation channels is 20-60. The ratio of current strength to electrolyte mass is 8-10. The ratio of the width of the electrolysis chamber to the width of the magnesium separation cell is 1.6-2.7. Additional channels are mounted in the partition, between the upper and lower circulation channels, said additional channels having a flow passage area of 0.016-0.048 of the area of the upper V-shaped channels.

FIELD OF INVENTION

The invention relates to the production of magnesium and chlorine from a magnesium chloride-containing salt melt by means of electrolysis.

BACKGROUND

The prior art discloses a method for the production of magnesium and chlorine from a MgCl₂-containing salt melt in an electrolytic cell having an electrolysis chamber with alternating anodes and cathodes and a magnesium separation cell separated from the electrolysis chamber by a partition having upper and lower circulation channels. The known method includes: maintaining a gas saturation of an electrolyte with chlorine bubbles in an interelectrode gap, which provides a closed circulation of the electrolyte between the electrolysis chamber and the magnesium separation cell and prevents the electrolyte from flowing downward in the interelectrode gap; and initiating an electrolyte and magnesium flow over the cathodes, the flow being directed toward the upper circulation channels. The known method is characterized in that the rate of the electrolyte and magnesium flow over the cathodes is adjusted by changing the height of the upper circulation channels depending on an average interelectrode distance, the height being selected from the following condition:

h _(channel)=(1.0÷10.0)·l _(average),

where h_(channel) is the height of the upper circulation channels, cm; l_(average) is the average interelectrode distance, cm.

The gas saturation of the electrolyte with chlorine bubbles in the interelectrode gap is maintained equal to 6-25 conventional units, which is determined by the following formula:

${G = \frac{d_{c} \cdot H_{c}}{I_{a\nu erage}}},$

where G is the gas saturation of the electrolyte with the chlorine bubbles in the interelectrode gap; d_(c) is the cathode current density, A/cm²; H_(c) is the cathode height, cm.

The electrolysis is carried out with a variable cross-sectional area of interelectrode gaps in the direction of upward electrolyte flows, the cross-sectional area increasing along the path of the melt.

RU 2243295 discloses an electrolytic cell for the production of magnesium and chlorine, which comprises an electrolysis chamber having alternating anodes and cathodes and a magnesium separation cell separated from the electrolysis chamber by a partition having upper and lower circulation channels. The electrolytic cell is characterized in that the cathode has a height which is 25-60 times higher than an interelectrode gap, and the cathode or anode has a working surface inclined at an angle of 38′÷1°26′ to the vertical.

The drawbacks of the electrolytic cell disclosed in RU 2243295 are that high rates of the electrolyte and magnesium flow coming from the electrolysis chamber to the magnesium separation cell are provided in the upper V-shaped circulation channels, thereby increasing the carryover of the chlorine bubbles from the electrolysis chamber to the magnesium separation cell, i.e. increasing the loss of chlorine with plumbing gases and worsening magnesium separation conditions in a collection cell. Small magnesium beads are carried away by downward electrolyte flows from the separation cell into the electrolysis chamber, thereby decreasing the current output of magnesium due to the reverse reaction of magnesium chlorination in the electrolysis chamber and increasing a specific electric energy consumption. During the operation of the electrolytic cell, the separating partition between the upper and lower circulation channels is under a cathodic potential due to magnesium and magnesium oxide outgrowths which are formed on the cathode and adjacent to the lining of the separating partition, thereby leading to the destruction of the separating partition and shortening the service life of the electrolytic cell.

RU 2220232 discloses an electrolytic cell for the production of magnesium, which comprises a bath lined with refractory materials, electrolytic compartments having graphite anodes alternating with steel cathodes, collection cells arranged parallel to the electrodes and separated from the electrolytic compartments by partitions. The electrodes in the electrolytic compartments are electrically connected in parallel, and the electrodes of each electrolytic compartment are connected to the electrodes of the adjacent compartment in series. The known electrolytic cell is characterized in that the collection cell arranged between the series-connected electrolytic compartments is arranged between dissimilar electrodes having the same electric potential, and an interelectrode distance in the electrolytic compartments is set to 20 mm-30 mm at a cathodic current density of 0.40 A/cm²-0.60 A/cm².

The drawback of the electrolytic cell disclosed in RU 2220232 is that the arrangement of the collection cell parallel to the electrodes lengthens the path of magnesium from the electrolysis chamber to the magnesium separation cell, thereby increasing the reverse interaction of magnesium with chlorine in the electrolysis chamber and decreasing the current output. The parallel electrical connection of the electrodes in the electrolysis chamber and the serial electrical connection of the electrodes between the electrolysis chambers significantly complicates the design of the electrolytic cell, thereby leading to current leakages between the chambers and decreasing the current output.

KZ 16980 discloses a method for the production of magnesium and chlorine from a MgCl₂-containing salt melt by using an electrolytic cell. The electrolytic cell includes an electrolysis chamber having alternating anodes and cathodes, a magnesium separation cell separated from the electrolysis chamber by a partition having upper V-shaped and lower circulation channels. The known method includes providing a closed circulation of an electrolyte between the electrolysis chamber and the magnesium separation cell due to the gas saturation of the electrolyte with chlorine bubbles in an interelectrode gap, which is equal to 6÷25. The gas saturation is determined by the following formula:

${\Pi = \frac{H_{c} \cdot d_{c}}{l_{a\nu erage}}},$

where Π is the gas saturation of the electrolyte with the chlorine bubbles in the interelectrode gap; d_(c) is the cathode current density, A/cm²; H_(c) is the cathode height, cm.

Upward electrolyte flows in the interelectrode gaps have a variable cross-sectional area that increases along the path of the melt, and an electrolyte flow rate in the upper V-shaped circulation channels is selected from the following condition:

${\frac{h_{channel}}{l_{a\nu erage}} = {1.0 \div 10.0}},$

where h_(channel) is the height of the upper circulation channels, cm; l_(average) is the average interelectrode distance, cm.

An electrolytic cell for implementing this known method includes an electrolysis chamber having alternating anodes and cathodes, a magnesium separation cell separated from the electrolysis chamber by a partition having upper V-shaped and lower circulation channels. The ratio of the interelectrode distance at the level of the upper edge of the cathode to that at the level of the lower edge of the cathode is 1.1-3, and the cathode or anode has a working surface inclined to at an angle of 38°÷1°26′ to the vertical. The ratio of a current strength (kA) to an electrolyte mass (t) in the known method was more than 10 units.

The disadvantages of the method disclosed in KZ 16980 and the device for its implementation include the low thermal inertia of the electrolyte, for which reason a violation of heat balance on the electrolytic cell occurs at the slightest change in the current output, thereby decreasing the current output. The presence of a high rate of electrolyte circulation through the upper V-shaped channels causes magnesium to be crushed in the separation cell into small beads which are carried away by downward electrolyte flows back to the electrolysis chamber, where magnesium is chlorinated. This leads to a decrease in the current output and an increase in the loss of chlorine with plumbing gases. During the operation of the electrolytic cell, the separating partition between the upper and lower circulation channels is under a cathodic potential due to magnesium and magnesium oxide outgrowths which are formed on the cathode and adjacent to the lining of the separating partition, thereby leading to the destruction of the separating partition and shortening the service life of the electrolytic cell.

SUMMARY

The problem solved by the invention is as follows: how to reduce a specific energy consumption by increasing a current output, while reducing the loss of chlorine with plumbing gases and increasing the service life of an electrolytic cell.

The technical result provided by the invention is a reduction of a specific energy consumption by improving magnesium separation conditions in a collection cell and, therefore, increasing a current output of magnesium, as well as an increase in the productivity of an electrolytic cell, a reduction in the loss of chlorine with plumbing gases and an increase in the service life of the electrolytic cell.

The technical result is achieved by the proposed method for the production of magnesium and chlorine from a MgCl₂-containing salt melt by using a diaphragm-less electrolytic cell. The method includes initiating a closed circulation of an electrolyte between an electrolysis chamber and a magnesium separation cell due to the gas saturation of the electrolyte with chlorine bubbles in an interelectrode gap, which is equal to 6÷25, as determined by the following formula:

${\Pi = \frac{H_{c} \cdot d_{c}}{l_{a\nu erage}}},$

where Π is the gas saturation of the electrolyte with the chlorine bubbles in the interelectrode gap; d_(c) is the cathode current density, A/cm²; H_(c) is the cathode height, cm.

The method is characterized in that a current strength (kA) and a electrolyte mass are set in such a quantitative expression that their ratio is 8-10, and an electrolyte flow from the electrolysis chamber enters the collection cell in two directions: through upper circulation channels and additional channels arranged between the upper circulation channels and lower circulation channels. An electrolyte flow rate in the upper V-shaped circulation channels is selected from the following condition:

${\frac{S_{{upper}\mspace{14mu}{channels}}}{S_{{additional}\mspace{14mu}{channels}}} = {20 \div 60}},$

where S_(upper channels) is the total area of the upper circulation channels; S_(additional channels) is the total area of the additional channels.

The diaphragm-less electrolytic cell for implementing the above-described method includes the electrolysis chamber having alternating anodes and cathodes, the magnesium separation cell separated from the electrolysis chamber by a partition with the upper V-shaped and lower circulation channels. The electrolytic cell is characterized in that the ratio of widths of the electrolysis chamber and the magnesium separation cell is 1.6÷2.7, the additional channels are provided in the partition between the upper V-shaped and lower channels, the flow passage area of the additional channels is 0.016÷0.048 of the area of the upper V-shaped channels, and the additional channels in the partition are made of a fusion cast crystalline mica material, namely fluorophlogopite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general view of a diaphragm-less electrolytic cell,

FIG. 2 shows a section taken along line A-A, and

FIG. 3 shows a section taken along line B-B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A diaphragm-less electrolytic cell comprises: a steel housing 1 lined internally with a refractory material 2; an electrolysis chamber 3 having alternating anodes 4 and cathodes 5, interelectrode gaps 6 and a shelter 7 from above; a magnesium separation cell 8; a partition 9 separating the electrolysis chamber 3 from the magnesium separation cell 8 and having upper V-shaped circulation channels 10, additional channels 11 lined with fluorophlogopite 12 and lower channels 13.

The electrolytic cell operates as follows:

After a melt is poured into the electrolytic cell, direct current is fed to the electrodes. At an electrolyte temperature of 655° C.-670° C., chlorine is released at the anodes (4), and magnesium is released at the cathodes (5). Due to the high gas saturation of an electrolyte with chlorine bubbles in the interelectrode gap (6), there are upward flows of the electrolyte together with magnesium and chlorine which is collected above the electrolyte in the electrolysis chamber (3) and then removed to a main chlorine pipeline. In the upper part of the interelectrode gap 6, most of the electrolyte together with magnesium enters the magnesium separation cell (8) through the upper V-shaped circulation channels (10). Another part of the electrolyte also enters the magnesium separation cell through the additional channels. Magnesium accumulates on the surface of the electrolyte in the separation cell (8) and is periodically selected by a vacuum ladle. The magnesium-free electrolyte in the cell (8) is directed downward, and the electrolyte flows then enter the interelectrode gaps (6) through the lower channels 13.

The ratio of widths of the electrolysis chamber and the magnesium separation cell is 1.6÷2.7. With this ratio, favorable conditions for magnesium settlement in the separation cell are provided.

The additional channels are mounted in the partition between the upper V-shaped and lower channels, and have a flow passage area of 0.016÷0.048 of the area of the upper V-shaped channels. This allows reducing the flow rate of the electrolyte and magnesium in the upper V-shaped channels, improving the conditions for magnesium settlement in the magnesium collection, reducing the removal of chlorine bubbles with the electrolyte flows from the electrolysis chamber to the collection cell, and, therefore, reducing the loss of chlorine with the plumbing gases and avoiding the contact between the cathodes and the partition.

The additional channels in the partition are made of a fusion cast crystalline mica material, namely fluorophlogopite, which allows increasing the resistance of the refractory separating partition and increasing the service life of the electrolytic cell.

TABLE Operating factors of the electrolytic cell For the electrolytic For the electrolytic cell according to cell disclosed in Factors the invention KZ 16980 Specific energy consumption 12960 13430 Service life of electrolytic cell, 48 40 months Current output of magnesium, % 85 82 Productivity gain, % 3.6 0 Loss of chlorine with plumbing 40 70 gases, kg/tMg

The method for the production of magnesium and chlorine from a MgCl₂-containing salt melt is performed as follows:

A melt comprising magnesium, sodium and potassium chlorides is poured into the electrolytic cell lined with a refractory material, and direct current is fed to the electrodes. By changing the current strength (kA) and the electrolyte mass (t) in the electrolytic cell, the ratio of the current strength to the electrolyte mass is set equal to 8÷10. At an electrolyte temperature of 655° C.-670° C., chlorine is released at the anodes, and magnesium is released at the cathodes. Due to the high gas saturation of the electrolyte with chlorine bubbles, there are upward flows of the electrolyte together with magnesium and chlorine in the interelectrode gap. Chlorine is collected over the electrolyte in the electrolysis chamber, from where it is removed into the main chlorine pipeline. Most of the electrolyte, together with magnesium in the upper part above the cathodes, is directed through the upper V-shaped circulation channels in the separating partition into the magnesium separation cell. Another part of the electrolyte is directed from the electrolysis chamber to the separation cell through the additional channels arranged in the separating partition between the upper and lower circulation channels. Due to the fact that the electrolyte flow from the electrolysis chamber into the collection cell is bifurcated into two flows, the electrolyte flow rate in the upper circulation channels decreases and is determined by the ratio S_(upper channels):S_(lower channels) equal to 20-60. Magnesium accumulates on the surface of the electrolyte in the magnesium separation cell and is periodically selected by the vacuum ladle. The magnesium-free electrolyte in the collection cell is directed downward and then directed through the lower channels of the separating partition to the interelectrode gaps of the electrolysis chamber.

The maximum current output is obtained at an electrolyte flow rate in the upper channels determined by the ratio S_(upper channels):S_(additional channels)=20-60.

The current strength on the electrolytic cell is set based on conditions for maintaining heat balance and observing the ratio of the current strength to the electrolyte mass which is equal to 8-10, and the electrolyte mass is regulated by changing the electrolyte level in the electrolytic cell.

The electrolyte flow rate in the upper V-shaped circulation channels is selected based on the following condition:

${\frac{S_{{upper}\mspace{14mu}{channels}}}{S_{{additional}\mspace{14mu}{channels}}} = {20 \div 60}},$

where S_(upper channels) is the total area of the upper circulation channels; S_(additional channels) is the total area of the additional channels, and this selection is caused by the fact that when the ratio is less than 20, the conditions for magnesium removal from the electrolysis chamber to the separation cell are worsened, thereby decreasing the current output by 1-2%; and when the ratio is more than 60, the conditions for magnesium settlement in the separation cell are worsened, and small magnesium beads having a diameter of less than 1.0 mm are carried by downward electrolyte flows back into the electrolysis chamber, thereby decreasing the current output by 1-2%.

The selection of the ratio of the current strength (kA) to the electrolyte mass (t), i.e. 8-10, is due to the fact that when the ratio of the current strength to the electrolyte mass is more than 10, the thermal inertia of the electrolyte in the electrolytic cell decreases sharply, and the electrolyte temperature rises rapidly with the slightest decrease in the current output, thereby decreasing the current output; and when the ratio is less than 8, specific heat loss per square meter for the bottom area of the electrolytic cell increases, thereby increasing the specific energy consumption of the electrolytic cell. The electrolyte mass is maintained by changing the electrolyte level in the electrolytic cell.

TABLE 1 Dependence of the current output of magnesium on the ratio of the current strength (kA) to the electrolyte mass (t) Ratio of current output (kA) Current output of to electrolyte mass (t): magnesium, % 6 ÷ 8 83  8 ÷ 10 85 10 ÷ 12 83

TABLE 2 Dependence of the current output of magnesium on the electrolyte flow rate Electrolyte flow rate in upper V-shaped circulation channels, which is expressed as the following ratio of the total area of the upper Current output of circulation channels to the total area of magnesium according additional channels to the invention, % 10-20 84.0 20-60 85.0 60-70 83.5

TABLE 3 Comparative factors of the method according to the invention and the method disclosed in KZ 16980 For the method For the method according to disclosed in Factors the invention KZ 16980 Current strength, kA 225.0 220.0 Ratio of current strength to 8-10 10-12 electrolyte mass, kA/t Electrolyte flow rate in upper 20 ÷ 60 S_(additional channels) = 0 circulation channels is governed by the ratio: $\frac{S_{{upper}\mspace{14mu}{channels}}}{S_{{additional}\mspace{14mu}{channels}}}$ Specific energy consumption, 12960 13430 kWh/tMg Service life of electrolytic cell, 48 40 months Current output of magnesium, 85 82 % Productivity, kgMg/day 2084 1966 Loss of chlorine with plumbing 40 70 gases, kg/tMg

By using the method according to the invention, there is a decrease in the specific energy consumption due to an increase in the current output and a decrease in the loss of chlorine with the plumbing gases.

The electrolytic cell according to the invention has an extended service life and provides a decrease in the loss of chlorine with the plumbing gases.

An economic result is as follows: a reduction in the specific energy consumption by 470 kWh/tones of magnesium (tMg), an increase in the current output of magnesium due to the improvement of magnesium removal from the electrolysis chamber into the collection cell and the improvement of the conditions for magnesium separation in the collection cell, an increase in the electrolytic cell productivity by 6.0%, a decrease in the loss of chlorine with the plumbing gases by 30 kg/tMg, and an increase in the service life of the electrolytic cell by 8 months. 

What is claimed is:
 1. A method for the production of magnesium and chlorine from a MgCl₂-containing salt melt by using a diaphragm-less electrolytic cell, wherein the method comprises initiating a closed circulation of an electrolyte between an electrolysis chamber having alternating anodes and cathodes and a magnesium separation cell due to a gas saturation of the electrolyte with chlorine bubbles in an interelectrode gap, which is equal to 6÷25 and defined by: ${\Pi = \frac{H_{c} \cdot d_{c}}{l_{a\nu erage}}},$ where Π denotes the gas saturation of the electrolyte with the chlorine bubbles in the interelectrode gap; d_(c) denotes a cathode current density, A/cm²; H_(c) denotes a cathode height, cm. wherein a current strength (kA) and an electrolyte mass are set in the electrolytic cell in such a quantitative expression that a ratio of the current strength (kA) to the electrolyte mass (t) is 8÷10, and an electrolyte flow from the electrolysis chamber enters a collection cell in two ways: through upper circulation channels and additional channels arranged between the upper circulation channels and lower circulation channels, wherein an electrolyte flow rate in the upper V-shaped circulation channels is selected from the condition: ${\frac{S_{{upper}\mspace{14mu}{channels}}}{S_{{additional}\mspace{14mu}{channels}}} = {20 \div 60}},$ where S_(upper channels) denotes a total area of the upper circulation channels; S_(additional channels) denotes a total area of the additional channels.
 2. A diaphragm-less electrolytic cell for performing the method according to claim 1, wherein the electrolytic cell comprises an electrolysis chamber having alternating anodes and cathodes, a magnesium separation cell separated from the electrolysis chamber by a partition with upper V-shaped and lower circulation channels, wherein a ratio of widths of the electrolysis chamber and the magnesium separation cell is 1.6÷2.7, additional channels are mounted in the partition between the upper V-shaped and lower channels, the additional channels have a flow passage area equal to 0.016÷0.048 of an area of the upper V-shaped channels, and the additional channels in the partition are made of a fusion cast crystalline mica material, namely fluorophlogopite. 